Explicit beamforming in a high efficiency wireless local area network

ABSTRACT

A first communication device transmits a plurality of training signals to a second communication device via a communication channel. The first communication device receives feedback generated at the second communication device based on the plurality of training signals. The feedback includes steering matrix information for a plurality of orthogonal frequency division multiplexing (OFDM) tones and (ii) additional phase information corresponding to channel estimates obtained for the plurality of OFDM tone. The first communication device constructs, based on the steering matrix information, a plurality of steering matrices corresponding to the plurality of OFDM tones, and compensates, using the additional phase information, the plurality of steering matrices to reduce phase discontinuities between the OFDM tones. The first communication device steers, using the compensated steering matrices, at least one transmission via the communication channel to the second communication device.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/176,934 (now U.S. Pat. No. 10,116,359), entitled “ExplicitBeamforming in a High Efficiency Wireless Local Area Network,” filed onJun. 8, 2016, which claims the benefit of U.S. Provisional PatentApplication Nos. 62/172,500, filed on Jun. 8, 2015, and 62/244,278,filed Oct. 21, 2015, both entitled “Explicit Beamforming Design forHE-MIMO with Compressed LTF.” All of the applications referenced aboveare incorporated herein by reference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication networks and,more particularly, to wireless local area networks that utilizeorthogonal frequency division multiplexing (OFDM).

BACKGROUND

Wireless local area networks (WLANs) have evolved rapidly over the pastdecade. Development of WLAN standards such as the Institute forElectrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g,and 802.11n Standards has improved single-user peak data throughput. Forexample, the IEEE 802.11b Standard specifies a single-user peakthroughput of 11 megabits per second (Mbps), the IEEE 802.11a and802.11g Standards specify a single-user peak throughput of 54 Mbps, theIEEE 802.11n Standard specifies a single-user peak throughput of 600Mbps, and the IEEE 802.11ac Standard specifies a single-user peakthroughput in the gigabits per second (Gbps) range. Future standardspromise to provide even greater throughputs, such as throughputs in thetens of Gbps range.

SUMMARY

In an embodiment, a method for beamforming training includes: receiving,at a first communication device from a second communication device via acommunication channel, a plurality of training signals; determining, atthe first communication device based on the plurality of trainingsignals, a plurality of channel matrices corresponding to a plurality oforthogonal frequency division multiplexing (OFDM) tones; generating, atthe first communication device based on the plurality of channelmatrices, feedback information for the plurality of OFDM tones, thefeedback information including (i) steering matrix information for theplurality of OFDM tones and (ii) additional phase informationcorresponding to the plurality of channel matrices for the plurality ofOFDM tones, the additional phase information for reducing phasediscontinuity across the OFDM tones in steered transmissions that are tobe subsequently transmitted from the second communication device to thefirst communication device; and transmitting the feedback informationfrom the first communication device to the second communication device.

In another embodiment, an apparatus comprises: a network interfacedevice has one or more integrated circuits. The one or more integratedcircuits are configured to: receive a plurality of training signalstransmitted be a communication device via a communication channel;determine, based on the plurality of training signals, a plurality ofchannel matrices corresponding to a plurality of orthogonal frequencydivision multiplexing (OFDM) tones; generate, based on the plurality ofchannel matrices, feedback information for the plurality of OFDM tones,the feedback information including (i) steering matrix information forthe plurality of OFDM tones and (ii) additional phase informationcorresponding to the plurality of channel matrices for the plurality ofOFDM tones, the additional phase information for reducing phasediscontinuity across the OFDM tones in steered transmissions that are tobe subsequently transmitted by the communication device; and transmitthe feedback information to the communication device.

In yet another embodiment, a method for beamforming training includes:transmitting, from a second communication device to a firstcommunication device via a communication channel, a plurality oftraining signals; receiving, at the second communication device from thefirst communication device, feedback generated at the firstcommunication device based on the plurality of training signals, whereinthe feedback includes (i) steering matrix information for a plurality oforthogonal frequency division multiplexing (OFDM) tones and (ii)additional phase information corresponding to channel estimates obtainedfor the plurality of OFDM tones; constructing, at the secondcommunication device based on the steering matrix information, aplurality of steering matrices corresponding to the plurality of OFDMtones; compensating, at the second communication device using theadditional phase information, the plurality of steering matrices toreduce phase discontinuities between the OFDM tones; and steering, usingthe compensated steering matrices, at least one transmission via thecommunication channel from the second communication device to the firstcommunication device.

In still another embodiment, a network interface device has one or moreintegrated circuits. The one or more integrated circuits are configuredto: transmit, to a communication device via a communication channel, aplurality of training signals; receive, from the communication device,feedback generated at the first communication device based on theplurality of training signals, wherein the feedback includes (i)steering matrix information for a plurality of orthogonal frequencydivision multiplexing (OFDM) tones and (ii) additional phase informationcorresponding to channel estimates obtained for the plurality of OFDMtones; construct, based on the steering matrix information, a pluralityof steering matrices corresponding to the plurality of OFDM tones;compensate, using the additional phase information, the plurality ofsteering matrices to reduce phase discontinuities between the OFDMtones; and steer, using the compensated steering matrices, at least onetransmission via the communication channel to the communication device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example wireless local area network(WLAN), according to an embodiment;

FIG. 2 is a diagram of an example physical layer (PHY) data unit,according an embodiment;

FIGS. 3A-3C are diagrams illustrating example orthogonal frequencydivision multiplexing (OFDM) tone spacing used with OFDM symbols of aPHY data unit, according to several embodiments;

FIG. 4 is a diagram of an example sounding packet, according to anembodiment;

FIG. 5 is a diagram of an example beamforming feedback, according to anembodiment;

FIG. 6 is a flow diagram of an example method for beamforming in acommunication channel, according to another embodiment; and

FIG. 7 is a flow diagram of an example method for beamforming in acommunication channel, according to another embodiment.

DETAILED DESCRIPTION

In embodiments described below, a wireless network device such as anaccess point (AP) of a wireless local area network (WLAN) transmits datastreams to one or more client stations. The AP is configured to operatewith client stations according to at least a first communicationprotocol. The first communication protocol is sometimes referred hereinas “high efficiency WiFi,” “HEW” communication protocol, or IEEE802.11ax communication protocol. In some embodiments, the WLAN supportsmultiple input multiple output (MIMO) communication in which the APand/or the client stations include more than one antenna, therebycreating a plurality of spatial (or space-time) streams over which datacan be transmitted simultaneously. In an embodiment in which the APemploys multiple antennas for transmission, the AP utilizes variousantennas to transmit the same signal while phasing (and amplifying) thissignal as it is provided to the various transmit antennas to achievebeamforming or beamsteering. In order to implement a beamformingtechnique, the AP generally requires knowledge of certaincharacteristics of the communication channel between the AP and the oneor more client stations for which a beamforming pattern is to becreated. To obtain channel characteristics, according to an embodiment,the AP transmits to a client station a sounding packet including anumber of training fields that allow the client station to accuratelyestimate the MIMO channel. The client station then transmits or feedsback, in some form, the obtained channel characteristics to the AP, forexample by including channel characteristic information in a managementor a control frame transmitted to the AP. Upon receiving, from one ormore of the client stations, information characterizing thecorresponding communication channels, the AP is generates desired beampatterns to be used in subsequent transmissions to one or more stations,in various embodiments.

FIG. 1 is a block diagram of an example wireless local area network(WLAN) 10, according to an embodiment. An AP 14 includes a hostprocessor 15 coupled to a network interface 16. In an embodiment, thenetwork interface 16 includes one or more integrate circuits (ICs)configured to operate as discussed below. The network interface 16includes a medium access control (MAC) processor 18 and a physical layer(PHY) processor 20. The PHY processor 20 includes a plurality oftransceivers 21, and the transceivers 21 are coupled to a plurality ofantennas 24. Although three transceivers 21 and three antennas 24 areillustrated in FIG. 1, the AP 14 includes other suitable numbers (e.g.,1, 2, 4, 5, etc.) of transceivers 21 and antennas 24 in otherembodiments. In some embodiments, the AP 14 includes a higher number ofantennas 24 than transceivers 21, and antenna switching techniques areutilized. In an embodiment, the MAC processor 18 is implemented on atleast a first IC, and the PHY processor 20 is implemented on at least asecond IC. In an embodiment, at least a portion of the MAC processor 18and at least a portion of the PHY processor 20 are implemented on asingle IC.

In an embodiment, the PHY processor 20 scrambles an MPDU (e.g., a PHYservice data unit) based on a scramble seed.

In various embodiments, the MAC processor 18 and the PHY processor 20are configured to operate according to a first communication protocol(e.g., a High Efficiency, HE, or 802.11ax communication protocol). Insome embodiments, the MAC processor 18 and the PHY processor 20 are alsoconfigured to operate according to a second communication protocol(e.g., according to the IEEE 802.11ac Standard). In yet anotherembodiment, the MAC processor 18 and the PHY processor 20 areadditionally configured to operate according to the second communicationprotocol, a third communication protocol, and/or a fourth communicationprotocol (e.g., according to the IEEE 802.11a Standard and/or the IEEE802.11n Standard).

The WLAN 10 includes a plurality of client stations 25. Although fourclient stations 25 are illustrated in FIG. 1, the WLAN 10 includes othersuitable numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations 25 invarious scenarios and embodiments. At least one of the client stations25 (e.g., client station 25-1) is configured to operate at leastaccording to the first communication protocol. In some embodiments, atleast one of the client stations 25 is not configured to operateaccording to the first communication protocol but is configured tooperate according to at least one of the second communication protocol,the third communication protocol, and/or the fourth communicationprotocol (referred to herein as a “legacy client station”).

The client station 25-1 includes a host processor 26 coupled to anetwork interface 27. In an embodiment, the network interface 27includes one or more ICs configured to operate as discussed below. Thenetwork interface 27 includes a MAC processor 28 and a PHY processor 29.The PHY processor 29 includes a plurality of transceivers 30, and thetransceivers 30 are coupled to a plurality of antennas 34. Althoughthree transceivers 30 and three antennas 34 are illustrated in FIG. 1,the client station 25-1 includes other suitable numbers (e.g., 1, 2, 4,5, etc.) of transceivers 30 and antennas 34 in other embodiments. Insome embodiments, the client station 25-1 includes a higher number ofantennas 34 than transceivers 30, and antenna switching techniques areutilized. In an embodiment, the MAC processor 28 is implemented on atleast a first IC, and the PHY processor 29 is implemented on at least asecond IC. In an embodiment, at least a portion of the MAC processor 28and at least a portion of the PHY processor 29 are implemented on asingle IC.

According to an embodiment, the client station 25-4 is a legacy clientstation, i.e., the client station 25-4 is not enabled to receive andfully decode a data unit that is transmitted by the AP 14 or anotherclient station 25 according to the first communication protocol.Similarly, according to an embodiment, the legacy client station 25-4 isnot enabled to transmit data units according to the first communicationprotocol. On the other hand, the legacy client station 25-4 is enabledto receive and fully decode and transmit data units according to thesecond communication protocol, the third communication protocol, and/orthe fourth communication protocol.

In an embodiment, one or both of the client stations 25-2 and 25-3, hasa structure that is the same as or similar to the client station 25-1.In an embodiment, the client station 25-4 has a structure similar to theclient station 25-1. In these embodiments, the client stations 25structured the same as or similar to the client station 25-1 have thesame or a different number of transceivers and antennas. For example,the client station 25-2 has only two transceivers and two antennas (notshown), according to an embodiment.

In various embodiments, the MAC processor 18 and the PHY processor 20 ofthe AP 14 are configured to generate data units conforming to the firstcommunication protocol and having formats described herein. In anembodiment, the MAC processor 18 is configured to implement MAC layerfunctions, including MAC layer functions of the first communicationprotocol. In an embodiment, the PHY processor 20 is configured toimplement PHY functions, including PHY functions of the firstcommunication protocol. For example, in an embodiment, the MAC processor18 is configured to generate MAC layer data units such as MPDUs, MACcontrol frames, etc., and provide the MAC layer data units to the PHYprocessor 20. In an embodiment, the PHY processor 20 is configured toreceive MAC layer data units from the MAC processor 18 and encapsulatethe MAC layer data units to generate PHY data units such as PHY protocoldata units (PPDUs) for transmission via the antennas 24. Similarly, inan embodiment, the PHY processor 20 is configured to receive PHY dataunits that were received via the antennas 24, and extract MAC layer dataunits encapsulated within the PHY data units. In an embodiment, the PHYprocessor 20 provides the extracted MAC layer data units to the MACprocessor 18, which processes the MAC layer data units.

The transceiver(s) 21 is/are configured to transmit the generated dataunits via the antenna(s) 24. Similarly, the transceiver(s) 21 is/areconfigured to receive data units via the antenna(s) 24. The MACprocessor 18 and the PHY processor 20 of the AP 14 are configured toprocess received data units conforming to the first communicationprotocol and having formats described hereinafter and to determine thatsuch data units conform to the first communication protocol, accordingto various embodiments.

In various embodiments, the MAC processor 28 and the PHY processor 29 ofthe client device 25-1 are configured to generate data units conformingto the first communication protocol and having formats described herein.In an embodiment, the MAC processor 28 is configured to implement MAClayer functions, including MAC layer functions of the firstcommunication protocol. In an embodiment, the PHY processor 29 isconfigured to implement PHY functions, including PHY functions of thefirst communication protocol. For example, in an embodiment, the MACprocessor 28 is configured to generate MAC layer data units such asMPDUs, MAC control frames, etc., and provide the MAC layer data units tothe PHY processor 29. In an embodiment, the PHY processor 29 isconfigured to receive MAC layer data units from the MAC processor 28 andencapsulate the MAC layer data units to generate PHY data units such asPPDUs for transmission via the antennas 34. Similarly, in an embodiment,the PHY processor 29 is configured to receive PHY data units that werereceived via the antennas 34, and extract MAC layer data unitsencapsulated within the PHY data units. In an embodiment, the PHYprocessor 29 provides the extracted MAC layer data units to the MACprocessor 28, which processes the MAC layer data units.

The transceiver(s) 30 is/are configured to transmit the generated dataunits via the antenna(s) 34. Similarly, the transceiver(s) 30 is/areconfigured to receive data units via the antenna(s) 34. The MACprocessor 28 and the PHY processor 29 of the client device 25-1 areconfigured to process received data units conforming to the firstcommunication protocol and having formats described hereinafter and todetermine that such data units conform to the first communicationprotocol, according to various embodiments.

In various embodiments, one or both of the AP 14 and the client device25-1 are configured to receive OFDM data units that include reducedlength MPDUs. In an embodiment, for example, the AP 14 maintains anassociation of a client station with an allocated sub-channel of theOFDM communication channel such that the AP 14 can generally identifywhich client station has transmitted an OFDM data unit based on thesub-channel on which the OFDM data unit was received. In anotherembodiment, the client station 25-1 maintains an association of the AP14 with the allocated sub-channel such that the client station 25-1 cangenerally identify which AP has transmitted an OFDM data unit based onthe sub-channel on which the OFDM data unit was received.

FIG. 2 is a diagram of a physical layer (PHY) data unit 200 that the AP14 is configured to transmit to one or more client stations 25 (e.g.,the client stations 25-1), according to an embodiment. In an embodiment,one or more client stations 25 (e.g., the client stations 25-1) are alsoconfigured to transmit data units the same as or similar to the dataunit 200 to the AP 14. The data unit 200 conforms to the HEcommunication protocol and occupies a 20 MHz bandwidth. Data unitssimilar to the data unit 200 occupy other suitable bandwidth such as 40MHz, 80 MHz, 160 MHz, 320 MHz, 640 MHz, for example, or other suitablebandwidths, in other embodiments. The data unit 200 is suitable for“mixed mode” situations, i.e. when the WLAN 10 includes a client station(e.g., the legacy client station 24-4) that conforms to a legacycommunication protocol, but not the first communication protocol. Thedata unit 200 is utilized in other situations as well, in someembodiments.

The data unit 200 includes a preamble 202 including a legacy shorttraining field (L-STF) 205, a legacy long training field (L-LTF) 210, alegacy signal field (L-SIG) 215, a first HE signal field (HE-SIG-A) 220,a second HE signal field (HE-SIG-B) 222, an HE short training field(HE-STF) 225, and M HE long training fields (HE-LTFs) 230, where M is aninteger. Each of the L-STF 205, the L-LTF 210, the L-SIG 215, theHE-SIG-A 220, the HE-SIG-B 222, the HE-STF 225, and the M HE-LTFs 230comprises one or more OFDM symbols. In some embodiments and/orscenarios, the data unit 200 also includes a data portion 240 having oneor more OFDM symbols. In some embodiments and/or scenarios, the dataunit 200 omits the data portion 240.

In the embodiment of FIG. 2, the data unit 200 includes one of each ofthe L-STF 205, the L-LTF 210, the L-SIG 215, the HE-SIG-A 220. In otherembodiments in which a data unit similar to the data unit 200 occupies acumulative bandwidth other than 20 MHz, each of the L-STF 205, the L-LTF210, the L-SIG 215 and HE-SIG-A 220 is repeated over a correspondingnumber of 20 MHz sub-bands of the whole bandwidth of the data unit, inan embodiment. For example, in an embodiment, the data unit occupies an80 MHz bandwidth and, accordingly, includes four of each of the L-STF205, the L-LTF 210, the L-SIG 215, the HE-SIG-A 220. In an embodiment inwhich a data unit similar to the data unit 200 occupies a cumulativebandwidth other than 20 MHz, the HE-SIG-B is repeated over acorresponding number of 20 MHz sub-bands of the whole bandwidth of thedata unit. In another embodiment in which a data unit similar to thedata unit 200 occupies a cumulative bandwidth other than 20 MHz, theHE-SIG-B 222 includes different channel-specific portions correspondingto different 20 MHz sub-bands of the whole bandwidth of the data unit,and the different channel specific portions are transmitted in parallelin the corresponding 20 MHz sub-bands of the whole bandwidth of the dataunit 200.

In some embodiments, the modulation of different 20 MHz sub-bandssignals is rotated by different angles. For example, in one embodiment,all OFDM tones within a first subband are rotated 0-degrees, all OFDMtones within a second subband is rotated 90-degrees, a third sub-band isrotated 180-degrees, and a fourth sub-band is rotated 270-degrees. Inother embodiments, different suitable rotations are utilized. Thedifferent phases of the 20 MHz sub-band signals result in reduced peakto average power ratio (PAPR) of OFDM symbols in the data unit 200, inat least some embodiments. In an embodiment, if the data unit thatconforms to the first communication protocol is an OFDM data unit thatoccupies a cumulative bandwidth such as 20 MHz, 40 MHz, 80 MHz, 160 MHz,320 MHz, 640 MHz, etc., the HE-STF, the HE-LTFs, the HE-SIG-B and the HEdata portion occupy the corresponding whole bandwidth of the data unit.

FIGS. 3A-3C are diagrams illustrating OFDM tone spacing used with OFDMsymbols of a data unit, such as the data unit 200 of FIG. 2A, in someembodiments. Turning first to FIG. 3A, a tone spacing 300 corresponds totone spacing defined in a legacy communication protocol. For example,the tone spacing 300 corresponds to the tone spacing defined in the IEEE802.11ac Standard, in an embodiment. In an embodiment, an OFDM symbolgenerated with the tone spacing 300 for a particular bandwidth isgenerated using an Inverse Digital Fourier Transform (IDFT) size thatresults in a tone spacing (TS) of 312.5 kHz in the particular bandwidth.For example, an OFDM symbol generated with the tone spacing 300 for a 20MHz bandwidth is generated using a 64 point IDFT, resulting in the tonespacing (TS) of 312.5 kHz, in an embodiment. Similarly, an OFDM symbolgenerated with the tone spacing 300 for a 40 MHz bandwidth is generatedusing a 128 point IDFT, an OFDM symbol generated with the tone spacing300 for an 80 MHz bandwidth is generated using a 256 point IDFT, an OFDMsymbol generated with the tone spacing 300 for a 160 MHz bandwidth isgenerated using a 512 point IDFT, etc., in an embodiment. Alternatively,in some embodiments, an OFDM symbol generated for at least some of thechannel bandwidths is generated using an IDFT size that results in atone spacing (TS) of 312.5 kHz in a sub-band of the entire bandwidth. Insuch embodiments, multiple sub-bands of the OFDM symbol are individuallygenerated using the IDFT size that results in the tone spacing (TS) of312.5 kHz in the individual sub-bands. For example, an OFDM symbol for a160 MHz-wide channel is generated using a 256 point IDFT in each one ofthe two 80 MHz sub-bands of the 160 MHz-wide channel, in an embodiment.

Turning now to FIG. 3B, a tone spacing 320 is reduced by a factor 2 (½)with respect to the tone spacing 300 of FIG. 3A. For example, continuingwith the example above, whereas on OFDM symbol generated with the tonespacing 300 for a 20 MHz bandwidth is generated using a 64 point IDFT,an OFDM symbol generated with the tone spacing 320 for a 20 MHzbandwidth is generated using a 128 point IDFT, resulting in the ½ of thetone spacing 300 of FIG. 3A (i.e., 156.25 kHz). Similarly, an OFDMsymbol generated with the tone spacing 320 for a 40 MHz-wide channel isgenerated using a 256 point IDFT, an OFDM symbol generated with the tonespacing 320 for an 80 MHz bandwidth channel is generated using a 512point IDFT, an OFDM symbol generated with the tone spacing 320 for a 160MHz bandwidth channel is generated using a 1024 point IDFT, etc., in anembodiment. Alternatively, in some embodiments, an OFDM symbol generatedfor at least some of the channel bandwidths is generated using an IDFTsize that results in a tone spacing (TS) of 156.25 kHz in a sub-band ofthe entire bandwidth. In such embodiments, multiple sub-bands of theOFDM symbol are individually generated with the IDFT size that resultsin the tone spacing (TS) of 312.5 kHz in the individual sub-bands. Forexample, an OFDM symbol for a 160 MHz bandwidth channel is generatedusing a 512 point IDFT in each one of the two 80 MHz sub-bands of the160 MHz bandwidth channel, in an embodiment.

Turning now to FIG. 3C, a tone spacing 350 is reduced by a factor 4 (¼)with respect to the tone spacing 300 of FIG. 3A. For example, continuingagain with the example above, whereas an OFDM symbol generated with thetone spacing 300 for a 20 MHz bandwidth is generated using a 64 pointIDFT, an OFDM symbol generated with the tone spacing 350 for a 20 MHzbandwidth is generated using a 256 point IDFT, resulting in the ¼ of thetone spacing 300 of FIG. 3A (i.e., 78.125 kHz), in an embodiment.Similarly, an OFDM symbol generated with the tone spacing 350 for a 40MHz bandwidth channel is generated using a 512 point IDFT, an OFDMsymbol generated with the tone spacing 350 for an 80 MHz bandwidthchannel is generated using a 1024 point IDFT, an OFDM symbol generatedwith the tone spacing 350 for a 160 MHz bandwidth channel is generatedusing a 2048 point IDFT, etc., in an embodiment. Alternatively, in someembodiments, an OFDM symbol generated for at least some of the channelbandwidths is generated using an IDFT size that results in a tonespacing (TS) of 78.125 kHz in a sub-band of the entire bandwidth. Insuch embodiments, multiple sub-bands of the OFDM symbol are individuallygenerated with the IDFT size that results in the tone spacing (TS) of312.5 kHz in the individual sub-bands. For example, an OFDM symbol for a160 MHz bandwidth channel is generated using a 512 point IDFT each oneof the 80 MHz sub-bands of the 160 MHz bandwidth channel, in anembodiment. As just another example, an OFDM symbol for a 40 MHzbandwidth channel is generated using a 256 point IDFT in each one of the20 MHz sub-bands of the 40 MHz bandwidth channel, in an embodiment. Asyet another example, in yet another embodiment, an OFDM symbol for an 80MHz bandwidth channel is generated using a 256 point IDFT in each one ofthe four 20 MHz sub-bands of the 80 MHz bandwidth channel, in anembodiment.

A tone spacing defined in a legacy communication protocol, such as thetone spacing 300 of FIG. 3A, is sometimes referred to herein as “normaltone spacing” and a tone spacing that is smaller than the tone spacingdefined by the legacy communication protocol, such as the tone spacing320 of FIG. 3B and the tone spacing 350 of FIG. 3C is sometimes referredto herein as “reduced tone spacing.”

Generally speaking symbol duration of an OFDM symbols, in time, isinversely proportional to the tone spacing used with the OFDM symbol.That is, if Δf corresponds to the tone spacing used with an OFDM symbol,then the time symbol duration of the OFDM symbol is T=1/Δf. Accordingly,a relatively smaller tone spacing used with an OFDM symbol results in arelatively larger symbol duration of the OFDM symbol, and vice versa, inan embodiment. For example, a tone spacing of Δf=312.5 kHz as in FIG. 3Aresults in an OFDM symbol duration of 3.2 μs, while a tone spacing ofΔf=156.25 kHz as in FIG. 3B results in an OFDM symbol duration of 6.4μs, in an embodiment. Further, a sampling rate at which a receivingdevice (e.g., a client station 25 or the AP 14) needs to sample the OFDMsymbol is inversely proportional to the IDFT size (number of points)used to generate the OFDM symbol. In particular, in an embodiment, ifN_(fft) is the IDFT size used to generate the OFDM symbol, then thesampling rate at which the receiving device needs to sample the OFDMsymbol is T/N_(fft), where T is the OFDM symbol duration (T=1/Δf).

In some embodiments, OFDM symbols of the data portion 240 of the dataunit 200 are generated with a first tone spacing and OFDM symbols oftraining fields (e.g., the HE-LTF fields 230) are generated with asecond tone spacing larger than the first tone spacing. For example, inan embodiment, OFDM symbols of the training fields 230 of the data unit200 are generated with the normal tone spacing, while the OFDM symbolsof the data portion 240 are generated with the ½ tone spacing or ¼ tonespacing, in an embodiment. In this embodiment, the reduced tone spacingused in the data portion 240 of the data unit 200 increases throughputby allowing transmission of more data symbols in each OFDM symbol of thedata portion 240. Further, using the normal tone spacing with thetraining fields 230 of the data unit 200 results in a reduced overheadcompared to a data unit in which the ½ tone spacing or the ¼ tonespacing is used with OFDM symbols of the training fields 230 as well asOFDM symbols of the data portion 240, in an embodiment.

In an embodiment, a receiving device that receives the data unit 200utilizes the training fields 230 of the data unit 200 to obtain channelestimates that are then used to decode data in the data portion 240 ofthe data unit 200. In an embodiment in which a greater tone spacing isused with OFDM symbols of the training fields as compared to OFDMsymbols of the data portion 240, the receiving device utilizes asuitable technique to obtain channel estimates for those OFDM tones inthe data portion 240 that are not present in OFDM symbols of thetraining fields training fields 230. For example, the receiving deviceutilizes interpolation, such as linear interpolation or another suitableinterpolation technique, to obtain channel estimates for those OFDMtones in the data portion 240 that are not present in OFDM symbols ofthe training fields training fields 230. As just an example, in anembodiment in which the normal tone spacing is used with OFDM symbols ofthe training fields 230 and the ½ tone spacing is used with OFDM symbolsof the data portion 240, the receiving device uses the training fields230 to directly obtain channel estimates for every other OFDM tone inthe data portion 240. The receiving device interpolates between channelestimates obtained for each pair of adjacent OFDM tones to obtainchannel estimates for the additional OFDM tone between correspondingpair of OFDM tones in the data portion 240, in an embodiment. As justanother example, in an embodiment in which the normal tone spacing isused with OFDM symbols of the training fields 230 and the ¼ tone spacingis used with OFDM symbols of the data portion 240, the receiving deviceuses the training fields 230 to directly obtain channel estimates forevery fourth OFDM tone in the data portion 240. The receiving deviceinterpolates between channel estimates obtained for each pair ofadjacent OFDM tones to obtain channel estimates for the additional threeOFDM tones between pairs of OFDM tones in the data portion 240, in anembodiment.

At least some techniques for obtaining channel estimates for OFDM tonesin the data portion 240 for which corresponding OFDM tones are missingin the training fields 230 rely on amplitude and phase continuity of thechannel response in the communication channel between the transmittingdevice and the receiving device, in various embodiments. For example,interpolation, such as linear interpolation, relies on continuity ofamplitude and phase of the channel response, in an embodiment. However,some discontinuities in the amplitude and/or phase in the communicationchannel exist, in at least some situations, in an embodiment. Forexample, beamforming used by the transmitting device to steertransmissions to the receiving device results in amplitude and/or phasediscontinuities in the communication channel, in at least someembodiments. Such discontinuities degrade receiver performance forexample because interpolated channel estimates do not accurately reflectthe communication channel, in an embodiment. As described in more detailbelow, such discontinuities are reduced using additional beamformingfeedback, in various embodiments.

Generally speaking, to perform transmit beamforming, the AP relies uponknowledge of the downlink channel between the AP and the client station.In an embodiment, the downlink channel knowledge is obtained throughexplicit beamforming, where the client device receives a sounding packetfrom the AP, develops channel estimates of the downlink channel based onthe sounding packet received from the AP, and transmits the channelestimates, or steering information determined based on the channelestimates, back to the AP. Explicit beamforming uses one of three typesof feedback channel descriptions, in various embodiments. With channelstate information (CSI) feedback, in an embodiment, the client stationestimates the channel (e.g., determines a channel matrix) based on thesounding packet received from the AP and feeds estimated channel matrixback to the AP, in an embodiment. With noncompressed steering matrixfeedback, in an embodiment, the client station, based on the channelestimate from a sounding packet from the AP, determines a steeringmatrix that is to be used at the AP. The client station then feeds thesteering matrix, without compression, back to the AP. With compressedsteering matrix feedback, a similar process occurs, but the steeringmatrix is fed back in a compressed form.

FIG. 4 is a block diagram of a sounding packet 400 that the AP 14 isconfigured to transmit to a client station 25 (e.g., the client station25-1) to sound the communication channel between the AP 14 and theclient station 25-1, according to an embodiment. In an embodiment, theclient station 25-1 utilizes the sounding packet 400 to obtain channelestimates and to generate feedback based on the channel estimates. Theclient station 25-1 transmits the feedback, generated based on thesounding packet 400, to the AP 14. The AP 14 receives the feedback,generated based on the sounding packet 400, from the client station25-1, and generates, based on the feedback, a steering matrix to be usedfor transmission to the client station 25-1 from the AP 14. The AP 14utilizes the steering matrix to transmit at least one data unit, such asthe data unit 200 of FIG. 2, to the client station 25-1, in anembodiment.

The sounding packet 400 is similar to the data unit 200 of FIG. 2 andincludes some of the same elements with the data unit 200 of FIG. 2. Thesounding packet 400 includes a plurality of LTFs 430, in an embodiment.In an embodiment, the LTFs 430 correspond to the LTFs 230 of the dataunit 200 of FIG. 2. In an embodiment, the LTFs 430 are generated with atone spacing that is different from the tone spacing used in the dataportion 240 of the data unit 200. For example, in an embodiment, whereasOFDM symbols of the data portion 240 of the data unit 200 are generatedwith the ½ tone spacing illustrated in FIG. 3B or the ¼ tone spacingillustrated in FIG. 3C, OFDM symbols of the LTFs 430 of the soundingpacket 400 are generated with the normal tone spacing illustrated inFIG. 3A. As another example, in another embodiment, whereas OFDM symbolsof the data portion 240 of the data unit 200 are generated with the ¼tone spacing illustrated in FIG. 3C, OFDM symbols of the LTFs 430 of thesounding packet 400 are generated with the ½ tone spacing illustrated inFIG. 3B. In other embodiments, OFDM symbols of the LTFs 430 of thesounding packet 400 and/or OFDM symbols of the data portion 240 of thedata unit 200 are generated suing other suitable tone spacings. In oneembodiment, OFDM symbols of the LTFs 430 of the sounding packet 400 andOFDM symbols of the data portion 240 of the data unit 200 are generatedwith a same tone spacing.

In an embodiment, the sounding packet 400 is a non-data packet (NDP)that omits a data portion. In another embodiment, the sounding packet400 includes a data portion.

The client station 25-1 receives the sounding packet 400 transmitted bythe AP 14, and determines a channel description based on the LTFs 430 ofthe sounding packet 400, in an embodiment. In an embodiment, the channeldescription includes channel gain parameters (which may be complexnumbers) for various streams within the communication channel betweenthe AP 14 and the client station 25-1. In some embodiments, the channeldescription is represented in a matrix form. For example, the channeldescription includes channel gain parameters (which may be complexnumbers) for various streams defined, at the one end, by the arrayincluding the antennas 24-1, 24-2, and 24-3 of AP 14 and, at the otherend, by the array including the antennas 34-1, 34-2, and 34-3 of theclient station 25-1, in an embodiment. In this embodiment, the channeldescription may be represented by a three-by-three channel matrix H thatspecifies, in each element, a channel gain parameter for a streamdefined by the corresponding transmit antenna and a receive antenna. Inan embodiment, the client station 25-1 determines a respective channeldescription (e.g., channel matrix) for each OFDM tone of the LTFs 430.Thus, channel description determined by the client station 25-1 includesmultiple channel matrices, each channel matrix corresponding to aparticular OFDM tone of the LTFs 430, in an embodiment.

Based the plurality of channel matrices H, the client station 25-1determines a plurality of beamforming feedback matrices V that are thenfed back, in some form (e.g., uncompressed or compressed form) fed backto the AP 14, in an embodiment. The client station 25-1 utilizes anysuitable technique for determining a beamforming feedback matrix V basedon a channel matrix H for each OFDM tone for which a channel matrix wasdetermined, in an embodiment. For example, the client station 25-1utilizes singular value decomposition (SVD) to decompose the channelmatrix H into a left singular value matrix, a diagonal singular valuematrix, and a right singular value matrix. In an embodiment, SVD of amatrix H_(i) corresponding to the i-th OFDM tone can be represented byH _(i) =U _(i)·Λ_(i) ·V _(i) ^(H)  Equation 1where V_(i) is the beamforming feedback matrix for the i-th OFDM tone.In other embodiments, other suitable techniques for generating steeringvectors based on channel estimates may be used in place of SVD.

FIG. 5 is a diagram of an example beamforming feedback 500 generated bythe client station 25-1, according to an embodiment. Generally speaking,beamforming feedback generated by the client station 25-1 includesinformation that will allow the AP 14 to construct steering matricesQ_(i) that can then be applied to steer transmission to the clientstation 25-1 to the AP 14. In an embodiment, the beamforming feedback500 includes steering matrix information 502 and additional phaseinformation 504. In various embodiments, the steering matrix information502 includes suitable representations of the beamforming matrices V_(i),in noncompressed or compressed form. In an embodiment in whichnoncompressed beamforming feedback is used, the steering matrixinformation 502 includes representations of elements of the beamformingmatrices V_(i). For example, in an embodiment the steering matrixinformation 502 includes quantized versions of the elements of thebeamforming feedback matrices V_(i).

In an embodiment in which compressed beamforming matrix is used, thesteering matrix information 502 includes quantized angles that representthe elements of the beamforming feedback matrices V_(i). In anembodiment, to compress a steering matrix V_(i), the client station 25-1rotates phases of elements of a row (e.g., the last row) of the steeringmatrix V_(i) such that the row of the steering matrix V_(i) becomenon-negative real numbers, i.e. with phases equal to zero. The clientstation 25-1 then decomposes the steering matrix V_(i) (withnon-negative real numbers in the rotated row) into a plurality ofmatrices that cause elements of columns of the steering matrix V tobecome non-negative real numbers. For example, to decompose the steeringmatrix V_(i), the client station 25-1 implements a Givens rotationalgorithm that iteratively rotates columns of the steering matrix V_(i)to cause columns of the steering matrix V_(i) (with non-negative realnumbers in the rotated row) to become non-negative real numbers, i.e.with phases equal to zero. In an embodiment, the client station 25-1generates the steering matrix information 502 to include representationsof rotation angles (e.g., ψ and φ angles) that result from thedecomposition of each steering matrix V_(i), in an embodiment. Forexample, the client station 25-1 generates the steering matrixinformation 502 to include quantized versions of rotation angles (e.g.,ψ angles and φ angles) that result from the decomposition of eachsteering matrix V_(i), in an embodiment.

The additional phase information 504 includes additional information tobe used by the AP 14 to compensate steering matrices generated at the AP14 based on the steering matrix information 502, according to anembodiment. In an embodiment, the AP 14 compensates steering matricesgenerated at the AP 14 based on the steering matrix information 502 toreduce phase discontinuities in the communication channel whentransmissions are steered to the client station 25-1 to the AP 14. In anembodiment, corresponding to each OFDM tone for which beamformingfeedback matrix information is included in the steering matrixinformation 502, the additional phase information 504 includes phasecomponents of elements of a row (e.g., the first row) of thecorresponding matrix U_(i) in Equation 1, in an embodiment. For example,in an embodiment, the additional phase information includes angles Φ_(i)calculated according toΦ_(i)=angle(U _(i) _(0,0) ,U _(i) _(0,1) , . . . ,U _(i) _(0,N-1))  Equation 2where U_(i) _(0,j) is the element in the first row and j-th column ofthe matrix U_(i), and where N is the number of columns in the matrixU_(i). Additionally, in an embodiment in which compressed beamformingfeedback is used, the additional phase information 504 includes,corresponding to each OFDM tone for which beamforming feedback matrixinformation is included in the steering matrix information 502, phasecomponents of elements of the row (e.g., the last row) of the matrixV_(i) corresponding to the OFDM tone. For example, in an embodiment, theadditional phase information includes angles θ_(i) calculated accordingtoθ_(i)=angle(V _(i) _(N-1,0) ,V _(i) _(N-1,1) , . . . ,V _(i) _(N-1,N-1))  Equation 3where V_(i) _(N-1,j) is the element in the last row and j-th column ofthe matrix V_(i), and where N is the number of columns in the matrixV_(i).

In some embodiments, the client station 25-1 performs additionalprocessing of the beamforming feedback matrices V_(i) before sending thebeamforming feedback matrix information to the AP 14. In suchembodiments, the steering matrix information 502 includes noncompressedor compressed representations of the processed beamforming feedbackmatrices V_(i). For example, the client stations 25-1 implements asmoothing technique to smooth beamforming feedback matrices V_(i) acrossa number of neighboring OFDM tones. In an embodiment, the client station25-1 implements smoothing with phase roll compensation to smooth thebeamforming feedback V_(i) over an integer number N of OFDM tonesaccording to

$\begin{matrix}{{V(n)} = {\sum\limits_{k = {{- {({N - 1})}}/2}}^{{({N - 1})}/2}\;{w_{k}{V\left( {n + k} \right)}e^{- {j{({k\;\alpha})}}}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$where w_(k) is a weighting factor, and the phase factor α is a phaseroll compensation factor. In an embodiment, the weighting factor w_(k)is set to 1/N. In this embodiment, beamforming feedback matrices areaveraged across the N OFDM tones. In other embodiments, other weightingfactors are used. In an embodiment in which phase roll compensation isnot used, the client station 25-1 implements smoothing using equation 4with the phase factor angle factor α set to zero. In other embodiments,other suitable values of the phase factor α are used.

The AP 14 receives the feedback from the client station 25-1, anddetermines, based on the received feedback, steering matrices Q_(i) tobe used for transmission to the client station 25-1, in an embodiment.In an embodiment, the AP 14 reconstructs steering matrices based on thesteering matrix information included in the feedback. In an embodiment,the AP 14 constructs an initial steering matrix Q_(0i) corresponding toeach OFDM tone for which beamforming feedback matrix is included in thebeamforming feedback received from the client station 25-1. The AP 14constructs the initial steering matrix Q_(0i) corresponding to aparticular OFDM based on the steering matrix information, included inthe beamforming feedback, corresponding to the particular OFDM tone, inan embodiment. For example, in the case of noncompressed feedback, theAP 14 reconstructs each beamforming feedback matrix V_(i) directly basedon the representation of the beamforming feedback matrix V_(i) includedin the feedback, and constructs the initial steering matrix Q_(0i) bysetting the initial steering matrix Q_(0i) to be equal to thereconstructed beamforming feedback matrix V_(i). In the case ofcompressed feedback, in an embodiment, the AP 14 reverses compressionprocess to reconstruct the uncompressed beamforming feedback matrixV_(i) based on representations of rotation angles (e.g., ψ angles and φangles) that resulted from the compression of the matrix V_(i), and setsthe initial steering matrix Q_(0i) to be equal to the correspondingreconstructed uncompressed beamforming feedback matrix V_(i).

In an embodiment, the AP 14 compensates the initial matrices Q_(0i)using the additional phase information 504 included in the feedback 500.In an embodiment, in the case of compressed beamforming feedback, the AP14 compensates the initial steering matrices Q_(0i) using the θ_(i)angles included in the additional phase information 504. In anembodiment, the AP 14 compensates the initial steering matrices Q_(0i)using the θ_(i) angles according toQ _(i) =Q _(0i)diag(e ^(jθ) ^(i) )  Equation 5

the AP 14 further compensates the steering matrices Q_(i) using Φ_(i)angles included in the additional phase information 504, in anembodiment. In an embodiment, the AP 14 further compensates the steeringmatrices Q_(i) using Φ_(i) angles according toQ _(i) =Q _(i)diag(d ^(jΦ) ^(i) )  Equation 6

In an embodiment in which the beamforming feedback matrix informationincludes noncompressed beamforming matrix information, the AP 14directly compensates the reconstructed steering matrices Q_(0i) usingΦ_(i) angles included in the additional phase information 504. In anembodiment, the AP 14 directly compensates the reconstructed steeringmatrices Q_(0i) using Φ_(i) angles according toQ _(i) =Q _(0i)diag(e ^(jΦ) ^(i) )  Equation 7

As discussed above, the AP 14 generates steering matrices Q_(i)corresponding to OFDM tones for which feedback information is includedin the feedback 500, in an embodiment. As also discussed above, a dataunit such as the data unit 200 that is to be steered based on thefeedback 500 includes additional OFDM tones corresponding to whichfeedback is not included in the feedback 500, in some embodiments. Forexample, whereas the feedback 500 is generated based on OFDM symbols ofLTFs 430 having a first tone spacing corresponding to a first number ofOFDM tones, OFDM tones of the data portion 240 are generated with asecond tone spacing that is less than the first tone spacing (e.g.,second tone spacing is ½ or ¼ of the first tone spacing) correspondingto a second number of OFDM tones that is greater than the first numberof OFDM tones, in some embodiments. In such embodiments, the AP 14generates steering matrices for the OFDM tones for which feedbackinformation is not included in the beamforming feedback 500 using thesteering matrices generated based on the beamforming feedback 500. Forexample, in an embodiment, the AP 14 utilizes interpolation tointerpolate between steering matrices Q_(i), corresponding to adjacentOFDM tones for which feedback was included in the beamforming feedback500. In an embodiment, linear interpolation is used to interpolatebetween steering matrices Q_(i). In an embodiment, linear interpolationwith phase roll compensation is used to interpolate between steeringmatrices Q_(i) For example, in an embodiment in which the second tonespacing used in the data portion 240 is ½ of the tone spacing used inthe OFDM symbols of the LTFs 430, the AP 14 interpolates betweensteering matrices Q_(i), corresponding to neighboring OFDM tones paccording toQ(2p+q)=[Q(2p)+Q(2p+2)·e ^(−jqw)]/2  Equation 8where q={0,1} and w is a phase roll.

As another example, in an embodiment in which the second tone spacingused in the data portion 240 is ¼ of the tone spacing used in the OFDMsymbols of the LTFs 430, the AP 14 interpolates between steeringmatrices Q_(i), corresponding to neighboring OFDM tones p according toQ(4p+q)=[Q(4p)+Q(4p+4)·e ^(−jqw)]/2  Equation 9where q={0, 1, 2, 3} and w is a phase roll compensation factor.

In some embodiments, interpolation is performed at the client station25-1 rather than at the AP 14. For example, the client station 25-1interpolates between beamforming feedback matrices V_(i) to generateadditional beamforming feedback matrices V_(i) corresponding to OFDMthat are to be used for transmission of data to the client station 25-1but that are missing in the LTFs 430 of the sounding packet 400 based onwhich beamforming feedback matrices V_(i) are generated by the clientstation 25-1, in an embodiment. For example, in some embodiments, theclient station 25-1 utilizes equations same as or similar to Equations 8and 9 to generate the additional beamforming feedback matrices byinterpolated between the beamforming feedback matrices V_(i) obtainedfor adjacent OFDM tones in OFDM symbols of the LTFs 430, in anembodiment.

The client station 25-1 includes representations of the additionalbeamforming feedback matrices V_(i) in the feedback 500, in anembodiment. For example, the steering matrix information of the feedback500 includes (i) representations of the beamforming feedback matricesV_(i) that are generated based of the LTFs 430 of the sounding packet400 and (ii) representations of the additional beamforming feedbackmatrices that generated based on (e.g., interpolated from) thebeamforming feedback matrices V_(i) that are generated based of the LTFs430 of the sounding packet 400, in some embodiments. In suchembodiments, the AP 14 constructs steering matrices corresponding toOFDM tones of OFDM symbols of a data portion of a data unit to betransmitted to the client station 25-1, such as the data portion 240 ofthe data unit 200, based on the steering matrix information 502 of thefeedback 500. In some such embodiments, the additional phase information504 is omitted from the steering matrix 500.

In some embodiments, to reduce amount of feedback transmitted from theclient stations 25-1 to the AP 14, the client station 25-1 transmitsbeamforming feedback information corresponding to only a subset of OFDMtones of OFDM symbols of a data portion of a data unit to be transmittedto the client station 25-1. For example, the client station 25-1transmits beamforming feedback information corresponding to one OFDMtone in each group of Ng adjacent OFDM tones of OFDM symbols of a dataportion of a data unit to be transmitted to the client station 25-1. Inan embodiment, the number Ng of OFDM tones in the group of OFDM tones isconfigurable. For example, in an embodiment, the client station 25-1configures the number Ng of OFDM tones in the group of OFDM tones. Inanother embodiment, the AP 14 configures the number Ng of OFDM tones inthe group of OFDM tones, and signals the number Ng to the client station25-1. In an embodiment, the number Ng of OFDM tones in the group of OFDMtones is configured based on one or more metrics associated with thecommunication channel between the client station 25-1 and the AP 14,such as channel conditions (e.g., signal to noise ratio), channelthroughput, error rate, etc. In some embodiments, the number Ng of OFDMtones in the group of OFDM tones is additionally or alternativelyconfigured based on capabilities of the client station 25-1 and/or ofthe AP 14.

In an embodiment, the AP 14 uses the generated steering matrices tosteer at least one transmission to the client station 25-1 to the AP 14.For example, in an embodiment, the AP 14 applies the steering matricesto OFDM tones of OFDM symbols in a data portion of a data unit, such asthe data portion 240 of the data unit 200, transmitted to the clientstation 25-1 to the AP 14.

In an embodiment, the client station 25-1 receives a data unit, such asthe data unit 200, to which beamforming was applied at the AP 14. Insome embodiments, the client station 25-1 utilizes the training fields230 of the data unit 200 to obtain a channel estimate that is then usedto decode data in the data portion 240 of the data unit 200. In anembodiment in which a greater tone spacing is used with OFDM symbols ofthe training fields as compared to OFDM symbols of the data portion 240,the receiving device utilizes a suitable technique to obtain channelestimates for those OFDM tones in the data portion 240 that are notpresent in OFDM symbols of the training fields training fields 230. Inan embodiment, the client station 25-1 utilizes a suitable smoothingtechnique to smooth channel estimates across multiple neighboring OFDMtones. For example, the client station 25-1 averages channel estimatescorresponding to the multiple OFDM tones. As discussed above, in atleast some embodiment, beamforming the data unit 200 causes phase and/oramplitude discontinuities when the data unit 200 is transmitted via acommunication channel. In some embodiments, the client station 25-1detects phase and/or amplitude jumps between adjacent OFDM tones, and toomit from channel smoothing those OFDM tones that experience phaseand/or amplitude jumps. For example, in an embodiment, the clientstation 25-1 omits from channel smoothing an OFDM tones for which adifference between phase and/or amplitude of channel estimatecorresponding to the OFDM tone and phase and/or amplitude of channelestimate corresponding to the adjacent OFDM tone exceeds a threshold.

FIG. 6 is a flow diagram of an example method 600 for beamforming in acommunication channel, according to an embodiment. With reference toFIG. 1, the method 600 is implemented by the network interface device16, in an embodiment. For example, in one such embodiment, the PHYprocessor 20 is configured to implement the method 600. According toanother embodiment, the MAC processing 18 is also configured toimplement at least a part of the method 600. With continued reference toFIG. 1, in yet another embodiment, the method 600 is implemented by thenetwork interface device 27 (e.g., the PHY processor 29 and/or the MACprocessor 28). In other embodiments, the method 600 is implemented byother suitable network interface devices.

At block 602, a plurality of training signals is received. In anembodiment, a sounding packet is received, wherein the sounding packetincludes the plurality of training signals. In an embodiment, thesounding packet 400 of FIG. 4 is received. In an embodiment, thetraining signals correspond to LTFs 403 of the sounding packet 400. Inanother embodiment, a suitable sounding packet different from thesounding packet 400 is received.

At block 604, a plurality of channel matrices corresponding to aplurality of OFDM tones are determined. In an embodiment, the pluralityof channel matrices are determined based on the plurality of trainingsignals received at block 602. In an embodiment, the plurality ofchannel matrices are determined based on LTFs of a sounding packetreceived at block 602. In an embodiment, the plurality of channelmatrices corresponds to the plurality of OFDM tones of OFDM symbols ofLTFs of a sounding packet received at block 602.

At block 606, beamforming feedback is generated based on the pluralityof steering matrices received at block 604. In an embodiment, thebeamforming feedback 500 is generated. In another embodiment,beamforming feedback different from the beamforming feedback 500 isgenerated. In an embodiment, the beamforming feedback informationincludes i) steering matrix information for the plurality of OFDM tonesand (ii) additional phase information corresponding to the plurality ofchannel matrices for the plurality of OFDM tones. In an embodiment, theadditional phase information is for reducing phase discontinuity acrossthe OFDM tones in steered transmissions that are to be subsequentlytransmitted by the communication device.

At block 608, the beamforming feedback is transmitted to thecommunication device.

FIG. 7 is a flow diagram of an example method 700 for beamforming in acommunication channel, according to an embodiment. With reference toFIG. 1, the method 700 is implemented by the network interface device16, in an embodiment. For example, in one such embodiment, the PHYprocessor 20 is configured to implement the method 700. According toanother embodiment, the MAC processing 18 is also configured toimplement at least a part of the method 700. With continued reference toFIG. 1, in yet another embodiment, the method 700 is implemented by thenetwork interface device 27 (e.g., the PHY processor 29 and/or the MACprocessor 28). In other embodiments, the method 700 is implemented byother suitable network interface devices.

At block 702, a plurality of training signals is transmitted to acommunication device. In an embodiment, a sounding packet istransmitted, wherein the sounding packet includes the plurality oftraining signals. In an embodiment, the sounding packet 400 of FIG. 4 istransmitted. In an embodiment, the training signals correspond to LTFs403 of the sounding packet 400. In another embodiment, a suitablesounding packet different from the sounding packet 400 is transmitted.

At block 704, feedback is received from the communication device. In anembodiment, the received feedback corresponds to feedback generated bythe communication device based the plurality of training signalstransmitted to the communication device at block 702. In an embodiment,the feedback received at block 704 corresponds to the feedback 500 ofFIG. 5. In another embodiment, the feedback received at block 704corresponds to suitable feedback different from the feedback 500 of FIG.5. In an embodiment, the feedback received at block 704 includes (i)steering matrix information for a plurality of OFDM tones and (ii)additional phase information corresponding to channel estimates obtainedfor the plurality of OFDM tones.

At block 706, a plurality of steering matrices corresponding to theplurality of OFDM tones is constructed. In an embodiment, the pluralityof steering matrices is constructed based on the feedback received atblock 704. In an embodiment, the plurality of steering matrices isconstructed based on the steering matrix information included in thefeedback received at block 704. At block 708, the steering matrices ofthe plurality of steering matrices constructed at block 706 arecompensated are compensated to reduce phase discontinuities across theplurality of OFDM tones. In an embodiment, the steering matrices of theplurality of steering matrices are compensated based on the additionalphase information included in the beamforming feedback received at block704.

At block 710, the plurality of compensated steering matrices is used tosteer at least one transmission to the communication device.

In an embodiment, a method for beamforming training includes: receiving,at a first communication device from a second communication device via acommunication channel, a plurality of training signals; determining, atthe first communication device based on the plurality of trainingsignals, a plurality of channel matrices corresponding to a plurality oforthogonal frequency division multiplexing (OFDM) tones; generating, atthe first communication device based on the plurality of channelmatrices, feedback information for the plurality of OFDM tones, thefeedback information including (i) steering matrix information for theplurality of OFDM tones and (ii) additional phase informationcorresponding to the plurality of channel matrices for the plurality ofOFDM tones, the additional phase information for reducing phasediscontinuity across the OFDM tones in steered transmissions that are tobe subsequently transmitted from the second communication device to thefirst communication device; and transmitting the feedback informationfrom the first communication device to the second communication device.

In other embodiments, the method includes one of, or any suitablecombination of two or more of, the following features.

Generating feedback information for a particular OFDM tone of theplurality of OFDM tones includes: decomposing a channel matrixcorresponding to a particular OFDM tone into a plurality of matricesincluding at least a first matrix and a second matrix, generating thesteering matrix information to include representations of elements ofthe first matrix, and generating the additional phase information toinclude phase components of at least some elements of the second matrix.

Generating the additional phase information comprises generating theadditional phase information to include phase components of elements ofa row of the second matrix.

The row of the second matrix is the first row of the second matrix.

Generating feedback information for a particular OFDM tone of theplurality of OFDM tones comprises: decomposing the channel matrixcorresponding to the particular OFDM tone into a plurality of matricesincluding at least a first matrix and a second matrix, compressing thesecond matrix to generate a plurality of angles, generating the steeringmatrix information to include representations of the plurality ofangles, and generating the additional phase information to include (i)representations of phase components of at least some elements of thefirst matrix and (ii) representations of phase components of at leastsome elements of the second matrix.

Generating the additional phase information the comprises generating theadditional phase information to include (i) representations of phasecomponents of elements of a first row of the first matrix and (ii)representations of phase components of elements a last row of the secondmatrix.

Generating feedback information for the plurality of OFDM tonesincludes: generating, based on the plurality of channel matrices, aplurality of beamforming feedback matrices corresponding to theplurality of OFDM tones, generating, based on the plurality ofbeamforming feedback matrices, a plurality of smoothed beamformingfeedback matrices corresponding to the plurality of OFDM tones,including generating a particular smoothed beamforming feedback matrixcorresponding to a particular OFDM tone using beamforming feedbackmatrices corresponding to OFDM tones within a window of N OFDM tonesaround the particular OFDM tones, wherein N is an integer greater thanone, and generating the steering matrix information feedback based onthe plurality of smoothed beamforming feedback matrices.

In another embodiment, an apparatus comprises: a network interfacedevice has one or more integrated circuits. The one or more integratedcircuits are configured to: receive a plurality of training signalstransmitted be a communication device via a communication channel;determine, based on the plurality of training signals, a plurality ofchannel matrices corresponding to a plurality of orthogonal frequencydivision multiplexing (OFDM) tones; generate, based on the plurality ofchannel matrices, feedback information for the plurality of OFDM tones,the feedback information including (i) steering matrix information forthe plurality of OFDM tones and (ii) additional phase informationcorresponding to the plurality of channel matrices for the plurality ofOFDM tones, the additional phase information for reducing phasediscontinuity across the OFDM tones in steered transmissions that are tobe subsequently transmitted by the communication device; and transmitthe feedback information to the communication device.

In other embodiments, the apparatus includes one of, or any suitablecombination of two or more of, the following features.

The one or more integrated circuits are further configured to: decomposea channel matrix corresponding to a particular OFDM tone into aplurality of matrices including at least a first matrix and a secondmatrix, generate the steering matrix information to includerepresentations of elements of the first matrix, and generate theadditional phase information to include phase components of at leastsome elements of the second matrix.

The one or more integrated circuits are configured to generate theadditional phase information to include phase components of elements ofa row of the second matrix.

The row of the second matrix is the first row of the second matrix.

The one or more integrated circuits are configured to: decompose thechannel matrix corresponding to the particular OFDM tone into aplurality of matrices including at least a first matrix and a secondmatrix, compress the second matrix to generate a plurality of angles,generate the steering matrix information to include representations ofthe plurality of angles, and generate the additional phase informationto include (i) representations of phase components of at least someelements of the first matrix and (ii) representations of phasecomponents of at least some elements of the second matrix.

The one or more integrated circuits are configured to generate theadditional phase information to include (i) representations of phasecomponents of elements of a first row of the first matrix and (ii)representations of phase components of elements a last row of the secondmatrix.

The one or more integrated circuits are further configured to: generate,based on the plurality of channel matrices, a plurality of beamformingfeedback matrices corresponding to the plurality of OFDM tones,generate, based on the plurality of beamforming feedback matrices, aplurality of smoothed beamforming feedback matrices corresponding to theplurality of OFDM tones, including generating a particular smoothedbeamforming feedback matrix corresponding to a particular OFDM toneusing beamforming feedback matrices corresponding to OFDM tones within awindow of N OFDM tones around the particular OFDM tones, wherein N is aninteger greater than one, and generate the steering matrix informationbased on the plurality of smoothed beamforming feedback matrices.

In yet another embodiment, a method for beamforming training includes:transmitting, from a second communication device to a firstcommunication device via a communication channel, a plurality oftraining signals; receiving, at the second communication device from thefirst communication device, feedback generated at the firstcommunication device based on the plurality of training signals, whereinthe feedback includes (i) steering matrix information for a plurality oforthogonal frequency division multiplexing (OFDM) tones and (ii)additional phase information corresponding to channel estimates obtainedfor the plurality of OFDM tones; constructing, at the secondcommunication device based on the steering matrix information, aplurality of steering matrices corresponding to the plurality of OFDMtones; compensating, at the second communication device using theadditional phase information, the plurality of steering matrices toreduce phase discontinuities between the OFDM tones; and steering, usingthe compensated steering matrices, at least one transmission via thecommunication channel from the second communication device to the firstcommunication device.

In other embodiments, the method includes one of, or any suitablecombination of two or more of, the following features.

The plurality of OFDM tones are spaced according to a first tone spacingcorresponding to a first number of OFDM tones in an OFDM symbol, andsteering the at least one transmission comprises using the compensatedsteering matrices to steer at least one OFDM symbol that corresponds toa second tone spacing corresponding to a second number of OFDM tone,wherein (i) the second tone spacing is smaller than the first tonespacing and (ii) the second number of OFDM tones is greater than thefirst number of OFDM tones.

The second number of OFDM tones is one of (i) twice the first number ofOFDM tone or (ii) four times the number of OFDM tones.

Steering the at least one OFDM symbol includes generating, based on thecompensated steering matrices, additional steering matrices for OFDMtones, of the at least one OFDM symbol, that are missing in theplurality of OFDM tones.

Generating, based on the compensated steering matrices, additionalsteering matrices for OFDM tones, of the at least one OFDM symbol, thatdo not correspond to OFDM tones in the plurality of OFDM tones comprisesinterpolating between compensated steering matrices corresponding to theplurality of OFDM tones.

Interpolating between compensated steering matrices corresponding to theplurality of OFDM includes compensating for phase roll between adjacentOFDM tones in the plurality of OFDM tones.

In still another embodiment, a network interface device has one or moreintegrated circuits. The one or more integrated circuits are configuredto: transmit, to a communication device via a communication channel, aplurality of training signals; receive, from the communication device,feedback generated at the first communication device based on theplurality of training signals, wherein the feedback includes (i)steering matrix information for a plurality of orthogonal frequencydivision multiplexing (OFDM) tones and (ii) additional phase informationcorresponding to channel estimates obtained for the plurality of OFDMtones; construct, based on the steering matrix information, a pluralityof steering matrices corresponding to the plurality of OFDM tones;compensate, using the additional phase information, the plurality ofsteering matrices to reduce phase discontinuities between the OFDMtones; and steer, using the compensated steering matrices, at least onetransmission via the communication channel to the communication device.

In other embodiments, the apparatus includes one of, or any suitablecombination of two or more of, the following features.

The plurality of OFDM tones are spaced according to a first tone spacingcorresponding to a first number of OFDM tones in an OFDM symbol, and theone or more integrated circuits are configured to steer, using thecompensated steering matrices, at least one OFDM symbol that correspondsto a second tone spacing corresponding to a second number of OFDM tone,wherein (i) the second tone spacing is smaller than the first tonespacing and (ii) the second number of OFDM tones is greater than thefirst number of OFDM tones.

The second number of OFDM tones is one of (i) twice the first number ofOFDM tone or (ii) four times the number of OFDM tones.

The one or more integrated circuits are further configured to: generate,based on the compensated steering matrices, additional steering matricesfor OFDM tones, of the at least one OFDM symbol, that are missing in theplurality of OFDM tones, and apply the additional steering matrices tocorresponding OFDM tones of the at least one OFDM symbol.

The one or more integrated circuits are configured to generate theadditional steering matrices for OFDM tones, of the at least one OFDMsymbol, by interpolating between compensated steering matricescorresponding to the plurality of OFDM tones.

Interpolating between compensated steering matrices corresponding to theplurality of OFDM includes compensating for phase roll between adjacentOFDM tones in the plurality of OFDM tones.

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. The software or firmware instructions mayinclude machine readable instructions that, when executed by one or moreprocessors, cause the one or more processors to perform various acts.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), a programmable logic device (PLD), etc.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, changes, additions and/or deletions may bemade to the disclosed embodiments without departing from the scope ofthe invention.

What is claimed is:
 1. A method for beamforming training, the methodcomprising: transmitting, by a first communication device, a pluralityof training signals to a second communication device via a communicationchannel; receiving, at the first communication device, feedbackgenerated at the second communication device based on the plurality oftraining signals, wherein the feedback includes (i) steering matrixinformation for a plurality of orthogonal frequency divisionmultiplexing (OFDM) tones and (ii) additional phase informationcorresponding to channel estimates obtained for the plurality of OFDMtones; constructing, at the first communication device i) based on thesteering matrix information and ii) without using the additional phaseinformation, a plurality of steering matrices corresponding to theplurality of OFDM tones; compensating, at the first communication devicebased on the additional phase information, the plurality of steeringmatrices to reduce phase discontinuities between the OFDM tones; andsteering, using the compensated steering matrices, at least onetransmission via the communication channel from the first communicationdevice to the second communication device.
 2. The method of claim 1,wherein: the plurality of OFDM tones are spaced according to a firsttone spacing corresponding to a first number of OFDM tones in an OFDMsymbol; and steering the at least one transmission comprises using thecompensated steering matrices to steer at least one OFDM symbol thatcorresponds to a second tone spacing corresponding to a second number ofOFDM tones, wherein (i) the second tone spacing is smaller than thefirst tone spacing and (ii) the second number of OFDM tones is greaterthan the first number of OFDM tones.
 3. The method of claim 2, whereinthe second number of OFDM tones is one of (i) twice the first number ofOFDM tones or (ii) four times the first number of OFDM tones.
 4. Themethod of claim 2, wherein steering the at least one OFDM symbolincludes generating, based on the compensated steering matrices,additional steering matrices for OFDM tones, of the at least one OFDMsymbol, that are missing in the plurality of OFDM tones.
 5. The methodof claim 4, wherein generating, based on the compensated steeringmatrices, additional steering matrices for OFDM tones, of the at leastone OFDM symbol, that do not correspond to OFDM tones in the pluralityof OFDM tones comprises interpolating between compensated steeringmatrices corresponding to the plurality of OFDM tones.
 6. The method ofclaim 5, wherein interpolating between compensated steering matricescorresponding to the plurality of OFDM tones includes compensating forphase roll between adjacent OFDM tones in the plurality of OFDM tones.7. An apparatus, comprising: a network interface device associated witha first communication device, wherein the network interface deviceincludes one or more integrated circuits configured to: transmit aplurality of training signals to a second communication device via acommunication channel, receive, from the second communication device,feedback generated at the second communication device based on theplurality of training signals, wherein the feedback includes (i)steering matrix information for a plurality of orthogonal frequencydivision multiplexing (OFDM) tones and (ii) additional phase informationcorresponding to channel estimates obtained for the plurality of OFDMtones, construct, i) based on the steering matrix information and ii)without using the additional phase information, a plurality of steeringmatrices corresponding to the plurality of OFDM tones, compensate, basedon the additional phase information, the plurality of steering matricesto reduce phase discontinuities between the OFDM tones, and steer, usingthe compensated steering matrices, at least one transmission via thecommunication channel to the second communication device.
 8. Theapparatus of claim 7, wherein: the plurality of OFDM tones are spacedaccording to a first tone spacing corresponding to a first number ofOFDM tones in an OFDM symbol; and the one or more integrated circuitsare configured to steer, using the compensated steering matrices, atleast one OFDM symbol that corresponds to a second tone spacingcorresponding to a second number of OFDM tones, wherein (i) the secondtone spacing is smaller than the first tone spacing and (ii) the secondnumber of OFDM tones is greater than the first number of OFDM tones. 9.The apparatus of claim 8, wherein the second number of OFDM tones is oneof (i) twice the first number of OFDM tones or (ii) four times the firstnumber of OFDM tones.
 10. The apparatus of claim 8, wherein the one ormore integrated circuits are further configured to: generate, based onthe compensated steering matrices, additional steering matrices for OFDMtones, of the at least one OFDM symbol, that are missing in theplurality of OFDM tones; and apply the additional steering matrices tocorresponding OFDM tones of the at least one OFDM symbol.
 11. Theapparatus of claim 10, wherein the one or more integrated circuits areconfigured to generate the additional steering matrices for OFDM tones,of the at least one OFDM symbol, by interpolating between compensatedsteering matrices corresponding to the plurality of OFDM tones.
 12. Theapparatus of claim 11, wherein the one or more integrated circuits areconfigured to, as part of interpolating between compensated steeringmatrices corresponding to the plurality of OFDM tones, compensate forphase roll between adjacent OFDM tones in the plurality of OFDM tones.13. A method for beamforming training, the method comprising:transmitting, by a first communication device, a plurality of trainingsignals to a second communication device via a communication channel;receiving, at the second communication device, the plurality of trainingsignals; determining, at the second communication device based on theplurality of training signals, a plurality of channel matricescorresponding to a plurality of orthogonal frequency divisionmultiplexing (OFDM) tones; generating, at the second communicationdevice based on the plurality of channel matrices, feedback informationfor the plurality of OFDM tones, the feedback information including (i)steering matrix information for the plurality of OFDM tones and (ii)additional phase information corresponding to the plurality of OFDMtones; and transmitting, by the second communication device, thefeedback information to the first communication device; receiving, atthe first communication device, the feedback information; constructing,at the first communication device, i) based on the steering matrixinformation and ii) without using the additional phase information, aplurality of steering matrices corresponding to the plurality of OFDMtones; compensating, at the first communication device based on theadditional phase information, the plurality of steering matrices toreduce phase discontinuities between the OFDM tones; and steering, usingthe compensated steering matrices, at least one transmission via thecommunication channel from the first communication device to the secondcommunication device.
 14. The method of claim 13, wherein: the pluralityof OFDM tones are spaced according to a first tone spacing correspondingto a first number of OFDM tones in an OFDM symbol; and steering the atleast one transmission comprises using the compensated steering matricesto steer at least one OFDM symbol that corresponds to a second tonespacing corresponding to a second number of OFDM tones, wherein (i) thesecond tone spacing is smaller than the first tone spacing and (ii) thesecond number of OFDM tones is greater than the first number of OFDMtones.
 15. The method of claim 14, wherein the second number of OFDMtones is one of (i) twice the first number of OFDM tones or (ii) fourtimes the first number of OFDM tones.
 16. The method of claim 14,wherein steering the at least one OFDM symbol includes generating, basedon the compensated steering matrices, additional steering matrices forOFDM tones, of the at least one OFDM symbol, that are missing in theplurality of OFDM tones.
 17. The method of claim 16, wherein generating,based on the compensated steering matrices, additional steering matricesfor OFDM tones, of the at least one OFDM symbol, that do not correspondto OFDM tones in the plurality of OFDM tones comprises interpolatingbetween compensated steering matrices corresponding to the plurality ofOFDM tones.
 18. The method of claim 17, wherein interpolating betweencompensated steering matrices corresponding to the plurality of OFDMtones includes compensating for phase roll between adjacent OFDM tonesin the plurality of OFDM tones.
 19. A system, comprising: a firstcommunication device comprising a first network interface device,wherein the first network interface device includes one or more firstintegrated circuits configured to: receive a plurality of trainingsignals transmitted by a second communication device via a communicationchannel, determine, based on the plurality of training signals, aplurality of channel matrices corresponding to a plurality of orthogonalfrequency division multiplexing (OFDM) tones, generate, based on theplurality of channel matrices, feedback information for the plurality ofOFDM tones, the feedback information including (i) steering matrixinformation for the plurality of OFDM tones and (ii) additional phaseinformation corresponding to the plurality of channel matrices for theplurality of OFDM tones, and transmit the feedback information to thesecond communication device; wherein the system further comprises thesecond communication device, the second communication device comprisinga second network interface device, wherein the second network interfacedevice includes one or more second integrated circuits configured to:transmit the plurality of training signals to the first communicationdevice via the communication channel, receive, from the firstcommunication device, the feedback information, construct, i) based onthe steering matrix information and ii) without using the additionalphase information, a plurality of steering matrices corresponding to theplurality of OFDM tones, compensate, based on the additional phaseinformation, the plurality of steering matrices to reduce phasediscontinuities between the OFDM tones, and steer, using the compensatedsteering matrices, at least one transmission via the communicationchannel to the first communication device.
 20. The system of claim 19,wherein: the plurality of OFDM tones are spaced according to a firsttone spacing corresponding to a first number of OFDM tones in an OFDMsymbol; and the one or more second integrated circuits are configured tosteer, using the compensated steering matrices, at least one OFDM symbolthat corresponds to a second tone spacing corresponding to a secondnumber of OFDM tones, wherein (i) the second tone spacing is smallerthan the first tone spacing and (ii) the second number of OFDM tones isgreater than the first number of OFDM tones.
 21. The system of claim 20,wherein the second number of OFDM tones is one of (i) twice the firstnumber of OFDM tones or (ii) four times the first number of OFDM tones.22. The system of claim 20, wherein the one or more second integratedcircuits are further configured to: generate, based on the compensatedsteering matrices, additional steering matrices for OFDM tones, of theat least one OFDM symbol, that are missing in the plurality of OFDMtones; and apply the additional steering matrices to corresponding OFDMtones of the at least one OFDM symbol.
 23. The system of claim 22,wherein the one or more second integrated circuits are configured togenerate the additional steering matrices for OFDM tones, of the atleast one OFDM symbol, by interpolating between compensated steeringmatrices corresponding to the plurality of OFDM tones.
 24. The system ofclaim 23, wherein the one or more second integrated circuits areconfigured to, as part of interpolating between compensated steeringmatrices corresponding to the plurality of OFDM tones, compensate forphase roll between adjacent OFDM tones in the plurality of OFDM tones.